Bud dormancy
As described earlier, bud dormancy is a
resting state of a stem bud in which the growth of the stem gets suspended
generally associated with the depression of axis elongation and the development
of a compact array of nodes enclosed in bud scales. The duration of bud
dormancy varies with different plants, from 2 to 8 weeks per cycle to a year or
more, depending upon the period of stress of drought, heat or cold. Thus the
dormancy of buds appears to be evolved from the tendency of many types of
perennials which show cyclic growth
The perennials
remain dormant during the period of environmental stress. Most of the temperate
trees require overwintering to get their buds released from dormancy while some
require proper photoperiod to break the dormancy. When a plant enters into
dormancy, it does not involve only the termination of active growth but
represents a morphologically distinct state. Some morphological and metabolic
states are-
Suspended
elongation of the axis,
Suspended
elongation of the leaf
Enclosement of
the shortened meristematic assemblage by distinctive scales.
Very low rate
of respiration,
Very low level
of nucleic acids.
Pollock (1953) showed
that respiration increase immediately after the removal of bud scales. it has been observed that the buds enter
dormancy by the repression of nucleic acid systems and their release from
dormancy is by derepression of this system. This system is geared up by the
environmental factors.
Wareing (1954)
showed that the bud dormancy could be imposed by exposing the plants to short
photoperiods. All the basic components of photoperiodic controls are present in
the bud scales of certain trees and the light and dark periods in the diurnal
cycle are involved in gearing dormancy.
Likewise the
cold exposure of plants also releases them from dormancy and buds start
sprouting. The natural exposure to low temperature is during the winter or
seasonal cold spells. The stimuli developed in response to low temperature are
accumulated in the buds which normally are expressed as an effect on the
quantitative increase in the growth of buds in response to the increasing
chilling duration. In some cases, the dormancy can be broken by heat treatment.
It was reported that inhibitors play an
important role in gearing bud dormancy. All the environmental stimuli which
induce dormancy do so by increasing the level of growth inhibitors. The
abscissic acid is the principal growth inhibitor present in dormant buds but in
some species some other types of inhibitors like phenanthrene, inhibitor b are also found.
Gibberellins
can overcome the effect of inhibitor and release the plant from dormancy.
Besides it the bud scales interfere with the growth of buds. These bud scales
compete with the meristematic tissues of the bud for their respiratory
activity. They have relatively very high rate of respiration as compared to the
enclosed meristematic tissue. Besides this, they have large amounts of
inhibitors. So when these bud scales are removed the meristematic tissue shows
an increase in their respiratory activity and at the same time the inhibitors
are also removed, then the buds show release of dormancy.
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